1. Field of the Invention
The present invention relates to a developing unit provided in an image forming apparatus of an electronic photography type and more particularly to a control unit for rotary components used for exhausting developing powder out of a developing bath in the developing unit.
2. Description of the Related Art
The inventors of the present application know a mechanism for adjustably feeding developing powder to a developing unit. The known mechanism includes a toner density sensor and is constructed so that when the sensor senses the absence of developing powder being recovered in the developing unit, the rotary system for recovering the developing powder is deactivated and the feeding unit is activated in response to the sensing signal, for feeding a necessary amount of new developing powder into the developing unit. This mechanism has been disclosed in Japanese Patent Lying Open No. 61-39061.
In the known mechanism, the rotary system such as rollers and screws provided in the developing unit is driven at a constant speed needed in a developing process. The same speed has been maintained in feeding new developing powder, exhausting waste developing powder, and developing an image.
As such, it may take a long time to exchange the developing powder. Since no work can be done for an image forming apparatus during the exchanging time, the long wasteful time is consumed in exchanging the developing powder.
As a prior art for solving the foregoing disadvantage, the applicant of the present application has proposed in the Japanese Patent Application No. 3-284558 that the rotary speed of the rollers and the screw given in exhausting the developing powder is faster than the speed given in developing an image.
In this prior art, the screw is constantly driven. Hence, the developing powder is forced to constantly travel toward an exhaust outlet, so that the developing powder is jammed around the exhaust outlet and causes the shaft of the screw to be locked with its bearing.